1. Field of the Invention
The present invention is directed to a luminescent storage screen suitable for storing a latent x-ray image, and in particular to a storage screen having a phosphor which is transparent in selected wavelength regions.
2. Description of the Prior Art
Luminescent storage screens are generally known in the art which consist of a stimulable phosphor in which a latent x-ray image is stored. Read-out of such stimulable phosphor screens is undertaken by exciting the phosphor of the screen with radiation at a first wavelength, thereby causing the screen to emit light at a second wavelength. Such a stimulable phosphor is disclosed, for example, in European application 0 174 875.
The use of such stimulable phosphors in an image pick-up device in a x-ray diagnostics installation is described, for example, in German patent 174 875, corresponding to U.S. Pat. No. 31847. In this known x-ray diagnostics installation, a luminescent storage screen, including a stimulable phosphor of the type described above, is irradiated with x-radiation which is attenuated by an examination subject. The stimulable phosphor serves as a radiation-sensitive transducer. As a result of the irradiation, electronic holes are generated in the phosphor, corresponding to the incident radiation intensity. These holes are stored in traps having a higher energy level, so that a latent x-ray image is thereby stored in the screen.
During read-out, the entire surface of the storage screen is caused to luminesce pixel-by-pixel by a separate radiation source such as, for example, a laser. The energy levels of the holes in the traps are boosted by the stimulating radiation, and thus fall back into lower energy levels, with the energy difference emitted in the form of light quanta. As a result, the stimulable phosphor emits light dependent on the energy stored in the phosphor. The light emitted due to the stimulation radiation is detected and converted into a visible image, so that the x-ray image stored in the screen can be read-out.
A problem in known stimulable phosphors used for this purpose is that the phosphor is not sufficiently transparent to the laser light. A minimum thickness of the stimulable phosphor is required to achieve adequate absorption of the x-ray quanta. In the case of a non-transparent, tightly compressed or sintered phosphor, the laser beam is so greatly attenuated by the phosphor that the penetration depth of the laser beam is very low. As the laser beam penetrates the phosphor, it relatively quickly reaches a depth at which its energy is no longer sufficient to boost the electronic holes to the energy level required for emission, so that the information stored in the deeper layers of the screen can not be read-out.
A storage screen is described in the aforementioned European application 0 174 87 wherein the grains of the phosphor of the storage screen are applied on a base enveloped by a binder. The binder serves to fix the phosphor grains. A light-transmissive carrier material is usually used as the binder, the carrier material being transparent both for the stimulating laser light and the emitted luminescent light. A problem still arises in that the laser beam spreads with increasing penetration depth in the phosphor, due to scatter caused by the phosphor grains, so that the modulation transfer function of the overall system is degraded. Compared to a layer consisting only of stimulable phosphor, a storage screen constructed in the binder technique having the same thickness, has a lower x-ray quantum absorption.